The present invention relates to a novel cyclone separator for removing fine solid particulates from a gas stream. The cyclone is especially applicable in a third stage separator apparatus, often used to purify the catalyst fines-laden flue gas stream exiting a refinery fluid catalytic cracking (FCC) catalyst regenerator.
The emission of particulates in industrial gas streams must be carefully controlled in light of federal, state, and local regulations designed to curtail pollution. In the area of oil refinery operations, a major concern regarding particulate emissions lies in the flue gas exiting the catalyst regenerator section of fluid catalytic cracking (FCC) units. Current United States federal regulations limit particulate levels to 1 lb. of solids per 1000 lb. of coke burned in the catalyst regenerator, or the equivalent of a flue gas particulate concentration of approximately 80 to 110 mg/Nm3. Corresponding European regulations currently vary considerably, from 80 to 500 mg/Nm3; however, this value is expected to decline potentially to 50 mg/Nm3.
FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. Although FCC is a large and complex process involving many factors, a general outline of the technology is presented here in the context of its relation to the present invention.
In its most general form, the FCC process comprises a reactor that is closely coupled with a catalyst regenerator, followed by downstream hydrocarbon product separation. A major distinguishing feature of the process is the continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50 to 100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, hence the considerable circulation requirements. Coupled with this need for a large inventory and recycle of a small particle diameter catalyst is the ongoing challenge to prevent this catalyst from exiting the reactor/regenerator system into effluent streams.
Overall, the use of cyclone separators internal to both the reactor and regenerator has provided over 99% separation efficiency of solid catalyst. Typically, the regenerator includes first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of the regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normal-sized catalyst particles are effectively removed in the internal regenerator cyclones, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor/regenerator environment) is substantially more difficult to separate. As a result, the FCC flue gas will usually contain a particulate concentration in the range of about 200 to 1000 mg/Nm3. This solids level can present difficulties related to either the applicable legal emissions standards or the desire to recover power from the flue gas stream. In the latter case, the solids content in the FCC flue gas may be sufficient to damage turbine blades of an air blower to the regenerator if such a power recovery scheme is indeed selected.
A further reduction in FCC flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS) device containing a manifold of cyclones. Electrostatic precipitators are known to be effective for this gas/solid separation but are far more costly than a TSS, which relies on the induction of centripetal acceleration to a particle-laden gas stream, forcing the higher-density solids to the outer edges of a spinning vortex. To be efficient, a cyclone separator for an FCC flue gas effluent will normally contain many, perhaps 100, small individual cylindrical cyclone bodies installed within a single vessel acting as a manifold. Tube sheets affixing the upper and lower ends of the cyclones act to distribute contaminated gas to the cyclone inlets and also to divide the region within the vessel into sections for collecting the separated gas and solid phases.
In the area of cyclone design, significant emphasis has been placed on so-called xe2x80x9creverse flowxe2x80x9d types where incoming gas is added around a gas outlet tube extending from the inlet side of a cylindrical cyclone body. Particle-rich gas can be withdrawn from openings in the sidewall of the cyclone body, while clean gas essentially reverses flow from its initial path toward the end of the cyclone body opposite the gas inlet, back toward the gas outlet. The gas outlet is a tube normally concentric with, and located within the cyclone body. These types of cyclones are described in U.S. Pat. No. 5,514,271 and U.S. Pat. No. 5,372,707, where the inventive subject matter is focused on the shape and distribution of the sidewall openings in order to minimize turbulent eddy formation that can re-entrain solids into the clean gas outlet. In U.S. Pat. No. 5,643,537 and parent U.S. Pat. No. 5,538,696, devices are contemplated for use with this fundamental cyclone design to further extend, or improve the uniformity of, the vortex flow pattern and thereby increase separation efficiency.
Unfortunately, the requirement by itself for a gas stream to reverse direction and exit the cyclone body on the same side as the gas inlet imposes flow disturbances that are not easily overcome. Cyclones of the type described in U.S. Pat. No. 5,690,709, termed xe2x80x9cuniflowxe2x80x9d, eliminate the re-entrainment of solids associated with the reversal of gas direction. In this case, clean gas moves continually downward and exits the cyclone body below a lower tube sheet, which serves as the physical boundary between the separated particles and purified gas. This design, however, also promotes non-uniform flow patterns, which are here associated with the discharge of particles at essentially right angles to the particle-laden gas vortex, through the open bottom in the cylindrical cyclone body. Again, the basic operation of the cyclone in this case involves a change in direction of gas flow that should ideally be avoided. Furthermore, the open bottom design provides a relatively large surface area for exiting xe2x80x9cdirtyxe2x80x9d gas to enter the bodies of adjacent cyclones in an overall arrangement of cyclones, such as in a TSS. This communication of gas among cyclones reduces separation efficiency.
Aside from general considerations about cyclone design, such as the induction of centripetal acceleration and the maintenance of a uniform flow pattern, further improvements in efficiency associated with any particular cyclone configuration must be verified through actual testing. Indeed, some proposed designs that were believed in principle to mitigate uneven flow patterns and localized eddy formation actually performed quite poorly in laboratory experiments. Even sophisticated computational fluid dynamics computer software has been found in some cases to be a poor predictor of TSS separation efficiency. Therefore, through extensive trial and error, coupled with the overall objective of refining the cyclone internal flow pattern, a significant improvement in fine particle separation from gas streams has been achieved.
The present invention is an improved cyclone for the separation of solid particulates from a gas stream. Many of these cyclones can be combined in a vessel for use as a third stage separator in the treatment of solid-contaminated gas streams, and in particular flue gas from a refinery fluid catalytic cracking unit or other solid-contaminated gas streams. The cyclone provides a high separation efficiency because a particulate-laden gas vortex is established and travels through the device with minimal flow pattern disturbances. The feed gas and exiting clean gas move in the same direction throughout the separation, and the clean gas, representing the bulk of the feed gas on a volume basis, is removed from the central portion of the vortex using a gas outlet tube extending with the cyclone body. Furthermore, solid particles are forced through openings in the sidewall of the cyclone body to prevent backflow and gas communication among adjacent cyclones, rather than discharged axially.
The use of a plate or other structure to close off the bottom of cyclone body means that particle-laden gas can exit only through openings on the cylinder wall. Thus, the pressure drop across the area through which the gas discharges is generally higher than that for open bottom designs. This increase in pressure drop and gas velocity induces a more forceful ejection of particulates through the cylinder sidewall, thereby preventing re-entry of solids into the cyclone body or any adjacent cyclones operating upon the same principal. In effect, the slots through which the particle-contaminated gas exits act as a xe2x80x9ccheck valvexe2x80x9d to prevent backflow and particle re-entrainment into the cyclone body.
The cyclone of the present invention is effective for separating even fine dust particles as small as 4 to 5 microns in diameter from the feed gas stream. These solid contaminants would otherwise render the contaminated gas non-compliant with environmental regulations or possibly prove detrimental to the proper functioning of power recovery turbines.
Accordingly, in one embodiment the present invention is a cyclone separator for location between an upper and a lower tube sheet in a third stage separator vessel. The cyclone comprises a substantially vertical cyclone body having a closed bottom end and a top end fixed with respect to the upper tube sheet. The cyclone body defines a feed gas inlet at its top end for receiving a particle-contaminated gas stream from above the upper tube sheet. A sidewall of the cyclone body defines a plurality of discharge openings between the upper and the lower tube sheets for tangentially discharging particles and a minor amount of an underflow gas stream. The cyclone of the present invention also comprises one or more swirl vanes proximate the gas inlet to induce centripetal acceleration of the particle-contaminated gas stream. The apparatus further comprises a gas outlet tube located centrally within the cyclone body, extending through the closed bottom, and further extending through the lower tube sheet. The gas outlet tube defines a clean gas inlet, usually above the discharge openings, for receiving a purified gas stream from within the cyclone body and further defines a clean gas outlet located below the lower tube sheet for discharging the purified gas stream.
In another embodiment, the present invention is a fluidized catalytic cracking process for cracking a heavy hydrocarbon feed. The process comprises contacting the heavy hydrocarbon feed with a cracking catalyst to produce a light hydrocarbon product and a spent catalyst having coke deposited thereon. The process further comprises regenerating the spent catalyst in a catalyst regenerator by contacting the spent catalyst with air to burn the coke and provide a regenerated catalyst and a flue gas. The process further comprises separating the regenerated catalyst from the flue gas using a first stage and a second stage separator located within the catalyst regenerator to yield a catalyst fines-contaminated flue gas stream. The process further comprises recycling the regenerated catalyst to the cracking reactor for further production of the light hydrocarbon product. Lastly, the process comprises purifying the catalyst fines-contaminated flue gas stream in a third stage separator apparatus having an upper and a lower tube sheet contained therein and a plurality of cyclones between the upper and lower tube sheets. In this embodiment, each cyclone comprises a substantially vertical cyclone body having a closed bottom end and a top end fixed with respect to the upper tube sheet. The cyclone body defines a feed gas inlet at its top end for receiving a particle-contaminated gas stream from above the upper tube sheet. A sidewall of the cyclone body defines a plurality of discharge openings between the upper and the lower tube sheets for discharging particles and a minor amount of an underflow gas stream. The cyclone of the present invention also comprises one or more swirl vanes proximate the gas inlet to induce centripetal acceleration of the particle-contaminated gas stream. The apparatus further comprises a gas outlet tube located centrally within the cyclone body, extending through the closed bottom, and further extending through the lower tube sheet. The gas outlet tube defines a clean gas inlet, usually located above the discharge opening, for receiving a purified gas stream from within the cyclone body and further defines a clean gas outlet located below the lower tube sheet for discharging the purified gas stream.